1. Field of the Invention
This invention relates to improved photothermographic materials and processes for preparing them. In one of its aspects it relates to certain photothermographic elements having improved photographic speed. In another of its aspects it relates to certain photothermographic compositions comprising photosensitive silver halide with a certain oxidation-reduction image-forming combination, a synthetic polymeric binder and certain photographic speed increasing compounds. In another of its aspects it relates to preparation of the described photothermographic materials.
2. Description of the State of the Art
Photothermographic materials and methods of their preparation are known. Such photothermographic materials after imagewise exposure can be heated to provide a developed image in the absence of separate processing solutions or baths. Typical photothermographic materials are described, for example, in U.S. Pat. No. 3,152,904 of Sorenson et al, issued Oct. 13, 1964; U.S. Pat. No. 3,457,075 of Morgan et al, issued July 22, 1969; U.S. Pat. No. 3,392,020 of Yutzy et al, issued July 9, 1968; British Specification 1,161,777 published Aug. 20, 1969; and U.S. Pat. No. 3,801,321 of Evans et al, issued Apr. 2, 1974.
It has been desirable to provide increased photographic speed for the photothermographic elements, as described, comprising (a) photosensitive silver halide, (b) an oxidation-reduction image-forming combination comprising (i) a long-chain fatty acid silver salt oxidizing agent, such as silver behenate or silver stearate, with (ii) a reducing agent, especially a sulfonamidophenol reducing agent, in (c) a synthetic polymeric binder, such as poly(vinyl butyral). Photographic speed as used herein is intended to mean the sensitivity to ranges of electromagnetic radiation to which the photothermographic materials are exposed imagewise to provide a latent image. Increased photographic speed is typically demonstrated by comparison of sensitometric curves, also known as H and D curves, resulting from sensitometric exposure of a photothermographic element followed by heating the element to develop the latent image. Photographic speed as used herein is expressed in terms of relative speeds in which a control is assigned a relative speed of 100. It has also been desirable in some cases to provide increased contrast and maximum density of the images produced with the described photothermographic materials.
One means which has been proposed to increase photosensitivity centers upon preparation of photosensitive silver halide in the photothermographic materials. This method provides what has been described as silver halide formation in situ. This is described, for example, in U.S. Pat. No. 3,457,075 of Morgan et al, issued July 22, 1969. Preparation of the photosensitive silver halide in situ is difficult to control. It is desirable to provide the photosensitive silver halide separate from other components of the photothermographic material and then mix the silver halide with the described components. This silver halide, prepared separately from other components of the photothermographic material, is referred to herein as ex situ silver halide. Photothermographic materials containing ex situ silver halide are described, for example, in U.S. Pat. No. 3,871,887 of Jones, issued Mar. 18, 1975. It has been desirable, however, to provide photographic speed in certain photothermographic materials beyond that provided alone by preparation of the silver halide either by in situ or ex situ methods.
Another means proposed for providing increased photographic speed in photothermographic materials has been the addition of onium halides to the photothermographic material as described, for example, in U.S. Pat. No. 3,679,422 of deMauriac et al, issued July 25, 1972. It has been found that mere addition of equivalent amounts of bromide ion or bromide compounds does not provide desired increase in sensitivity. Also, mere addition of silver iodide to a photothermographic composition, as described, has not been found to provide the desired increased photographic speed.
It is also known that a variety of mercapto heterocyclic compounds can be useful in photographic materials, such as photographic emulsions and processing solutions. These mercapto heterocyclic compounds have not been used in photothermographic materials to provide increased photographic speed. For example, spectral sensitizing dyes containing a mercapto heterocyclic moiety have been added to photographic materials in order to increase spectral sensitivity, but not to increase photographic speed of photothermographic materials as described herein. Certain mercapto heterocyclic compounds have been found useful in photothermographic materials, for example, for providing post-processing image stability as described, for example, in U.S. Pat. No. 3,839,041 of Hiller, issued Oct. 1, 1974. Mercaptotetrazole compounds have also been found useful for certain purposes in photothermographic materials as described, for example, in U.S. Pat. No. 3,666,477 of Goffe, issued May 30, 1972. Certain 2-mercaptosubstituted heterocyclic compounds have been used to provide chemical sensitization of photographic gelatino silver halide emulsions also. This is described, for example, in U.S. Pat. No. 3,785,822 of Overman, issued Jan. 15, 1974. It is also known to add certain thiazoline-2-thione compounds to conventional photographic silver halide emulsions to provide increased light sensitivity and improved black image tone. This is described, for example, in U.S. Pat. No. 3,433,640 of Nishio et al, issued Mar. 18, 1969. This description, however, does not relate to photothermographic materials containing an oxidation-reduction image-forming combination.
A variety of spectral sensitizing dyes are known to be useful in photothermographic materials as described in the above patents. Useful spectral sensitizing dyes are described, for example, in U.S. Pat. No. 3,761,279 of deMauriac et al, issued Sept. 25, 1973 and include merocyanine dyes, for instance, containing a rhodanine, thiohydantoin or 2-thio-2,4-oxazolidinedione nucleus. Spectral sensitizing dyes as a class, however, have not been found to provide the desired increased photographic speed, increased maximum density and contrast characteristics of the described photothermographic elements containing photosensitive silver halide with an oxidation-reduction image-forming combination. This is illustrated, for example, by the comparative data in U.S. Pat. No. 3,761,279.
Accordingly, there has been a continuing need to provide improved photothermographic materials and processes for preparing such materials using photosensitive silver halide with an oxidation-reduction image-forming combination comprising a long-chain fatty acid silver salt oxidizing agent with a reducing agent, especially a sulfonamidophenol reducing agent.